1. Field of the Invention
The present invention relates to a laser system which allows asynchronous modelocking of two wavelengths in a single gain medium with mutually controllable repetition rates.
2. Description of the Related Art
In general, work in the field of dual-wavelength operation of modelocked solid-state lasers has centered on laser systems based on either bulk Ti:sapphire lasers or erbium fiber lasers. Such laser systems are attractive since they can be used in nonlinear optical mixing techniques or laser sensor systems. Particularly when the lasers operate asynchronously they can be used to make pump probe measurements.
However, a problem common to both Ti:sapphire and erbium is that they are homogeneously broadened. As a result, there is gain-competition between the two wavelengths, therefore requiring an exact balancing of the cavity losses to prevent the domination of one wavelength. Specifically, in Ti:sapphire the coupling between the two wavelengths is so strong that asynchronous operation of the two wavelengths in a single gain medium is not possible.
For example, M. R. X. de Barros and P. C. Becker, in Opt. Lett., 18, 631 (1993) describe the simultaneous generation of two collinear synchronous modelocked pulse trains, with wavelength separation of 60 to 80 nm, from a single cavity modelocked Ti:sapphire laser. In this laser, pulse synchronization is caused by the modelocking process being stronger with spatial and temporal overlap between the two pulses in the Ti:sapphire crystal. The temporal overlap provides for coupling between the two colors, which promotes the locking of the two pulses in synchronous mode. If the pulses do not overlap, e.g., if the two cavity lengths are not equal, there is competition between the two colors and one of them will be extinguished. Thus, stable operation of the laser in a dual wavelength mode is not achievable.
In addition, D. R. Dykaar and S. B. Darack, in Opt. Lett., 18, 634 (1993) describe the generation of dual-wavelength femtosecond pulses in a single cavity modelocked Ti:sapphire laser through the use of modified end mirrors to produce a second beam. Synchronization of the pulses is maintained by crossing the beams of the argon pump lasers and matching the cavity lengths of each wavelength. However, since the beams do not overlap, they can be allowed to operate asynchronously. By using the same pump laser and crystal, jitter between the two pulses can be minimized.
However, neither of the above described references produces dual-wavelength operation in asynchronous mode. As discussed above, such asynchronous operation is difficult to achieve in homogeneous gain media.
In addition, the use of a single gain medium would be greatly preferred so as to obtain the lowest possible phase noise between the two emission wavelengths. However, this is possible only when the wavelengths are weakly coupled. Even though modelocked erbium fiber lasers operate asynchronously, current erbium fiber lasers do not allow for an accurate wavelength selection anywhere in the cavity, which prevents control over the two-wavelength operation state.